Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 28 2023 19:51:35 FOUS30 KWBC 281951 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Sat Oct 28 2023 Day 1 Valid 16Z Sat Oct 28 2023 - 12Z Sun Oct 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF NORTHEAST TEXAS THROUGH THE LOWER OHIO VALLEY...=20 ....16Z Update... ....Texas... The showers and storms that brought as much as 14 inches of rain to areas just east of Dallas has both weakened and moved off to the north and east of the hardest hit areas around the Dallas-Fort Worth Metroplex this morning. This has left most of the area behind the slug of rain as mostly dry. This trend will continue for the rest of the day and into the start of the overnight hours, which should allow for some time for the rivers to drain off the excess rainfall. Tonight, expect renewed scattered showers and thunderstorms to develop across much of north/northeast TX, but the organization and instability for that rainfall activity is expected to be greatly degraded from what was seen last night, so a repeat flooding situation is highly unlikely again tonight at this point. With that said, however, due to the extremely low FFGs in the hardest hit areas, even the stronger showers and storms may briefly produce rainfall rates that could quickly cause additional isolated flash flooding, especially if 2 or more storms move over the same area in rapid succession. Given the above scenario, the Moderate Risk area was dropped with this update from Dallas south and east, but the much larger and broader Slight risk area remains in place with no changes planned through tonight for those scattered showers and storms. A few CAMs suggested that showers could develop into the TX Hill Country, so the Marginal risk was extended to the west to account for that possibility since that area has been wet was well in recent days. ....Arkansas through the Lower Ohio Valley... The area of moderate to heavy rain that inundated Texas is now tracking across Arkansas this midday. The rainfall rates have come down, the storms are moving, and much of Arkansas is experiencing dry to very dry soils and river levels. Thus, the situation across the state is night and day different from that over Texas. Further, instability values are meager across the area, generally under 500 J/kg of MUCAPE, which has contributed to the overall lower rainfall rates. Since atmospheric conditions are unlikely to change much from that description, think the heavy rainfall, and thus flash flooding threat is notably lower in this region than across Texas. Much of the area will see today's rainfall as largely beneficial, with the only flash flooding concerns in more highly sensitive or urbanized areas. The same holds true further north and east into the mid-Mississippi River and lower Ohio river valleys. Thus, while the Slight Risk area is largely unchanged with this update, the Slight is considered on the lower end of the threat category. If the rainfall threat from TX to AR continues tracking northeast as forecast in many of the CAMs, there should be a decent break from any steady rains across the area this evening into tonight, and the Slight may be able to be downgraded to a Marginal later this afternoon. Wegman ....Previous Discussion... =20 ....Northeast TX into the Lower Ohio Valley...=20=20 Precipitable water values of 1.5-2.2" are expected near a slow-moving front draped across the region ahead of a strengthening positively tilted upper level trough. The degree of low-level inflow and effective bulk shear available should continue to organize what should be ongoing convection Saturday morning into training bands as the flow is fairly uniformly out of the west-southwest with height. MU CAPE of 500+ J/kg should be sufficient to achieve hourly rain totals to 2.5" at times, which would be a problem across area terrain, which should slowly lower with time as instability slowly lessens. Local amounts in the 3-5" range are indicated by the available guidance overall. The Slight Risk area from continuity looked within reason which was shifted minimally based upon the most recent model guidance, though extension farther back into northeast TX to accommodate the new Moderate Risk area -- where spots have picked up 8"+ of rain overnight -- appeared wise given ongoing rainfall and the threat for additional heavy rains early on.=20 =20=20 Roth Day 2 Valid 12Z Sun Oct 29 2023 - 12Z Mon Oct 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST TEXAS INTO THE MID-SOUTH... ....20Z Update... No significant changes were made to the ERO risk areas with this update. There remains some uncertainty as to the flood threat from Arkansas into the mid-Mississippi Valley, as the Day 2/Sunday flood threat in these areas will be highly dependent on how much rain falls during the Day 1/Saturday period. The area has been very dry lately, and there remains very little to no instability for the rainfall to feed off of. The area has been really dry prior to today/Saturday, so it will take quite a bit of rain between today and Sunday to cause widely scattered impacts. The greatest chance of those impacts is across central to northeastern AR. The broader Slight covers the hard hit areas of north-central and northeast TX, and in similar fashion, the mid-Mississippi and lower Ohio Valleys, though the flood threat is low here as well. More scattered convection with a bit better instability will be present across the TX Hill Country on down to the Big Bend area, but that would result in widely scattered footprints of heavier rain, not a pattern that is overly favorable to flash flooding. Wegman ....Previous Discussion... =20 ....Southern Plains into the Ohio Valley...=20 Precipitable water values will be in slow decline this period, maximizing above 1.75" early on before falling into the 1.25-1.5"range by Monday morning. A positively-tilted upper level trough will be approaching the region from the west and north causing some forward progression to the front across Texas and the Lower Mississippi Valley with time. The degree of low-level inflow and effective bulk shear available could organize convection into training bands during the day Sunday, assuming sufficient instability, as the flow is fairly uniformly out of the west-southwest with height, but should be fading somewhat Sunday night into Monday morning. MU CAPE of 250-500 J/kg should be sufficient to achieve hourly rain totals of 1-2" at times early on, which would be a problem across area terrain and places that rained heavily on preceding days, but the 00z NAM indicates the instability fades with time on Sunday and Sunday night. Local amounts in the 2-4" range are indicated by the available guidance overall. The biggest change to continuity was to extend the Slight Risk area farther southwest, which has as much to do with current soil saturation and ongoing heavy rainfall as it does with the possibility of additional heavy rainfall. There was some temptation to raising the threat level to a Moderate Risk, particularly early in the period when the parameters for heavy rainfall look best which would account equally for heavy rainfall continuing into Sunday morning and how much rain fell the preceding day in and near AR, but the minimal instability forecast was the biggest drawback. =20 Roth Day 3 Valid 12Z Mon Oct 30 2023 - 12Z Tue Oct 31 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Some consideration was given to a targeted Marginal south and east of Dallas, TX where lingering rainfall from the overnight will continue on Day 3/Monday, but amounts should almost universally remain under a half inch. Despite the low FFGs, those amounts should not be enough to cause flash flooding. It's likely the FFGs may slowly come up with more time under no or even light rain. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6F82rqUj5H3Dicowhkj9A1QMBUerbmJvVXn7agFRL7yg= lfSLXBn16clQHWNexXu8u-uVv3jpaVYdtc09yLWLQq17dVc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6F82rqUj5H3Dicowhkj9A1QMBUerbmJvVXn7agFRL7yg= lfSLXBn16clQHWNexXu8u-uVv3jpaVYdtc09yLWLSpskUg0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6F82rqUj5H3Dicowhkj9A1QMBUerbmJvVXn7agFRL7yg= lfSLXBn16clQHWNexXu8u-uVv3jpaVYdtc09yLWLlmpLmoI$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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